The Atari 8-bit family is a series of 8-bit home computers manufactured from 1979 to 1992. All are based on the MOS Technology 6502 CPU and were the first home computers designed with custom coprocessor chips. Over the following decade several versions of the same basic design were released, including the original Atari 400 and 800 and their successors, the XL and XE series of computers. Overall, the Atari 8-bit computer line was a commercial success, selling two million units through its major production run between late 1979 and mid-1985, a total of around 4 million units.

Design of the 8-bit series of machines started at Atari Inc. as soon as the Atari 2600 games console was released in late 1977. The engineering team from Atari Grass Valley Research Center (originally "Cyan Engineering") felt that the 2600 would have about a three year lifespan before becoming obsolete. They started "blue sky" designs for a new console that would be ready to replace it around 1980, three years after the 2600's introduction. What they ended up with was essentially a "corrected" version of the 2600, fixing its more obvious limitations. The newer design would be faster than the 2600, have better graphics, and would include much better sound hardware. Work on the chips for the new system continued throughout 1978 and focused on much-improved video hardware known as the Alphanumeric Television Interface Controller, or ANTIC and the Color Television Interface Adaptor, or CTIA.

During this gestation the home computer era began in earnest in the form of the TRS-80, Commodore PET, and Apple II family—what Byte Magazine would dub the "1977 Trinity". Ray Kassar, the then-new CEO of Atari from Warner Communications, wanted the new chips to be used in a home computer to challenge Apple. In order to adapt the machine to this role, it would need to support character graphics, include some form of expansion for peripherals, and run the then-universal BASIC programming language. The need for character graphics led to the introduction of the ANTIC, a co-processor built to generate conventional bitmap graphics and characters providing a number of different modes with varying color support and resolution. Like the earlier TIA of the 2600, the CTIA was designed to produce Player-Missile graphics (sprites) and expanded to provide the color for the ANTIC's playfield graphics. ANTIC and CTIA work in concert to produce the complete display.

An overarching goal for the new computer systems was user-friendliness. The Atari computers were designed to minimize handling of bare circuit boards or chips common with upgrades or even initial set up of other systems of that period. The computers were designed with enclosed modules for memory, ROM cartridges, and keyed connectors. The system did not require the user enter commands to boot the system. The OS, large and comprehensive for its time, would boot automatically loading drivers from devices on the serial bus (SIO). The DOS system for managing floppy storage was menu driven. When no software was loaded, rather than leaving the user at a blank screen or machine language monitor, the OS would go to the "Memo Pad" mode allowing the user to type using the built-in full screen editor.
Atari had originally intended to port Microsoft BASIC to the machine, as had most other vendors, intending to supply it on an 8 KB ROM cartridge. However the existing 6502 version from Microsoft was 12 KB, and all of Atari's attempts to pare it down to 8 KB failed. Eventually they farmed out the work to a local consulting firm, Shepardson Microsystems, who recommended writing their own version from scratch, which was eventually delivered (on external cartridge) as Atari BASIC.

The machines were announced in late 1979 as the 400 and 800, although they were not widely available until November 1979, much closer to the original design date. The names originally referred to the amount of memory, 4 KB RAM in the 400 and 8 KB in the 800. However by the time they were released the prices on RAM had started to fall, so the machines were instead both released with 8 KB. As memory prices continued to fall Atari eventually supplied the 800 fully expanded to 48 KB, using up all the slots. Overheating problems with the memory modules eventually led Atari to remove the module's casings, leaving them as "bare" boards. Later, the expansion cover was held down with screws instead of the easier to open plastic latches. The Atari 400, despite its membrane keyboard and single internal ROM cartridge slot, outsold the full keyboard and RAM expandable Atari 800 by a 2-to-1 margin. Because of this, developers were generally unwilling to use the 800-only right cartridge slot.

The 400 and 800 were complex and expensive machines to build, consisting of multiple circuit boards mostly enclosed by massive die-cast aluminum shielding. Additionally, the machine was designed to add RAM only through cards, though it soon shipped fully expanded right from the factory. Soldering that RAM to the motherboard would be much less expensive than the connectors and separate cards needed in the 800. At the same time the 400 did not compete technically with some of the newer machines appearing in the early 1980s, which tended to ship with much more RAM and an upgraded keyboard. Another major change was the introduction of the FCC ratings specifically for digital devices in homes and offices. One of the ratings, known as Class B, mandated that the device's RF emissions were to be low enough not to interfere with other devices, such as radios and TVs. Now computers needed just enough shielding to prevent interference (both ways), not prevent any emissions from leaking out. This requirement enabled lighter, less expensive shielding than the previous 400 and 800 computers.

In 1982 Atari started the Sweet 8 (or "Liz NY") and Sweet 16 projects to take advantage of these changes. The result was an upgraded set of machines otherwise similar to the 400 and 800, but much easier to build and less costly to produce. Whereas the previous machines had individual circuit boards mounted inside and outside the internal shield, in the new design a single board supported all of the circuitry and the much thinner shielding was attached to it. This reduction in complexity was helped by improvements in chip making since the original machines were released, allowing a number of separate chips in the original systems to be condensed into one. Atari also ordered a custom version of the 6502, initially labeled "6502C" but eventually known as SALLY to differentiate it from a standard 6502C, which added a single pin that allowed four support chips to be removed. The SALLY was incorporated into late-production 400/800 machines, all subsequent XL/XE machines and Atari 5200/7800 game systems. Like the earlier machines, the Sweet 8/16 was intended to be released in two versions as the 1000 with 16 KB and the 1000X with 64 KB; RAM was still expensive enough to make this distinction worthwhile. In order to support expansion for high-end systems, similar to the card slots used in the Apple II or S-100 machines, the 1000 series also supported the Parallel Bus Interface (PBI), a single expansion slot on the back of the machine. An external chassis could be plugged into the PBI, supporting card slots for further expansion.

Additional products in the 8-bit line included the 800XL, 800XE and 1200XL.

On January 1, 1992, Atari corp. officially dropped all remaining support of the 8-bit line.

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